Integrated circuit gain-programmable amplifiers are employed in a wide range of electronic equipment, such as within wireless communications receivers, medical equipment, radar, and so forth. A typical gain-programmable amplifier includes an operational amplifier (op-amp) with a variable feedback resistor that is varied by the user or manufacturer to establish a fixed gain or to dynamically control the gain during amplifier operation.
FIG. 1 illustrates a conventional differential type of gain-programmable amplifier 10. A differential input voltage (Vip-Vin) is applied to the amplifier and amplified by op-amp 16 is to produce a differential output voltage (Vop-Von)=G(Vip-Vin), where G is the overall gain of the amplifier 10. The gain is controllable by varying the resistance of a pair of resistor networks R.sub.S1, R.sub.S2 coupled to the respective op-amp input terminals, and by varying the resistance of a pair of feedback resistor networks R.sub.F1, R.sub.F2. Each variable resistor network consists of a number of parallel-connected resistors R, with each resistor selectively connected to the parallel circuit via an associated switch S. Thus, by selectively switching the individual switches S on or off, the effective resistance of each resistor network is controlled to thereby influence the amplifier gain. The number of gain settings of amplifier 10 equals the total number of switches in all four resistor networks R.sub.S1, R.sub.S2, R.sub.F1 and R.sub.F2.
One shortcoming of the amplifier of FIG. 1 is that in practice, the circuit topology limits the amplifier bandwidth as well as the gain setting resolution. Increasing the number of settings to enhance gain resolution requires increasing the number of switches and resistors in the resistor networks. The larger number of switches and resistors, however, wastes silicon area and also results in a reduction of the amplifier bandwidth.
Amplifiers with digitally controllable gains are also in the prior art. See, e.g., U.S. Pat. No. 5,451,901 to D. Welland, which discloses an automatic gain control circuit employing a series connection of a transconductance amplifier having a digitally controllable transconductance, and a dynamically variable resistance circuit. The combination approximates an exponential response to a gain control signal.